In the prior arts, there have been several liquid level detecting device, one of which is adapted to photo-electrically detect a given level of liquid, and another of which is adapted to electrically detect a level of liquid using an electrical conductivity of liquid. However, in the former device, photoelectric converting means tends to be tainted by liquid to thereby operate the device in an erroneous manner, and in the latter device, detecting electrodes tend to be covered with insulation such as oil to thereby operate the device also in an erroneous manner.
In another prior art, there has been a device for electrically detecting a level of liquid using an electrically heating and thermally sensitive resistor means disposed flush with a liquid level to be detected. Such a device has been disclosed by U.S. Pat. No. 3,222,578 dated on Dec. 7, 1965, in which a liquid is of liquefied gas. This device utilizes a difference between the temperature t.sub.0 when the thermally sensitive resistor means is not in contact with the liquid and the temperature t.sub.1 when the resistor means is in contact with the liquid. In the device, the difference of temperature (t.sub.0 -t.sub.1) is required to be more than a range of variation in the temperature of environments. Therefore, if it is required that the range of variation in the temperature of environments for making the detection of liquid level possible is larger, then the temperature t.sub.0 when the resistor means is not in contact with the liquid is required to be substantially high. For example, if it is required that the detection of liquid level is possible for the environment temperature of 0.degree. to 60.degree. C., it reaches the temperature of more than 120.degree. C. Thus, it will be noted that the temperature of the thermally sensitive resistor means should be too high. This causes consuming electric power to be disadvantageously and ineconomically high.